1. Field of the Invention
The present invention relates to a method for measuring the concentration of a trace metal contained in a sample by measuring a change in absorbance caused by a complex forming reaction between a specific porphyrin-nucleus introduced polymer and a metal ion. More specifically, the invention pertains to a method capable of conveniently measuring the concentrations of many heavy metals simultaneously.
2. Discussion of the Background
Heavy metals are classified as xe2x80x9cindustrial wastes requiring specific managementxe2x80x9d and are subject to various environmental quality standards. There is a demand for a method of easily measuring the concentration of each of various heavy metals existing in rivers, lakes, marshes, sewage or factory effluent. It is common practice to employ atomic absorption spectrometry or inductively coupled plasma emission spectrometry (ICP) to quantitatively determine the concentration of heavy metals. These methods are not satisfactory for general purposes, because they involve problems such as high cost and the necessity of pipe arrangements or exhaust systems. On the other hand, absorption spectrophotometry does not have a cost problem and permits measurement through a relatively simple apparatus. However, because absorption spectrophotometry can be influenced by many substances that exist in a sample but are not intended to be measured, absorption spectrophotometry often requires a pretreatment of the sample to remove these substances.
Porphyrin compounds are colorimetric reagents used in absorption spectrophotometry. To their porphyrin ring, several metals are bonded, thereby forming complexes. These metals are affected by, as well as external factors such as pH, a substituent or functional group on the molecule of the porphyrin compounds.
As a calorimetric reagent, porphyrin compounds involve problems such as poor solubility in water. For improving their properties including water solubility or reactivity with metals, attempts have been made to substitute the reaction center, and to introduce a substituent or functional group in the vicinity thereof, bringing about great effects. In recent years, synthesis of water soluble porphyrins having various substituents or functional groups have been reported (J. Amer. Chem. Soc., 94 (1972) 4511; J. Amer. Chem. Soc., 93 (1971) 3162; Inorg. Chem., 9(1970)1757: J. Amer. Chem. Soc., 91(1969)607; and J. Heterocycl. Chem., 6(1969)927). There is also a report on the application of these porphyrins to analysis of metals contained at a trace level (ppb level) by making use of a change in fluorescence after formation of a complex with various metals (Analytica Chimica Acta, 74 (1975)53-60; J. Japan Chemical Soc., 1978, (5), 686-690; J. Japan Chemical Soc., 1979, (5), 602-606; BUNSEKI KAGAKU vol. 25 (1976) 781; Japanese patent application laid-open No. 082285/95).
Under the above-described techniques, however, it is necessary to introduce substituents or functional groups to a porphyrin ring in order to impart the porphyrin compound with water solubility or to change reactivity with a metal. An object of the present invention is therefore to develop a technique capable of imparting desired properties or reactivity to easily available porphyrin compounds without any modification, and to provide a highly sensitive method for measuring heavy metals by making use of the technique.
With the foregoing in view, the present inventors have found that by performing radical copolymerization of a specific porphyrin compound and a radically polymerizable monomer, thereby introducing the porphyrin nucleus into the main chain of the polymer having various side chains and, upon copolymerization, selecting the proper radically polymerizable monomer, it is possible to alter the stereostructure in the periphery of the porphyrin nucleus without using a porphyrin derivative in which various substituents or functional groups are introduced in the periphery of the porphyrin nucleus and hence to change water solubility of the resulting porphyrin-nucleus introduced polymer or reactivity with a metal upon complex formation. They have also found that highly sensitive and convenient measurement of a target metal can be attained easily by the use of the above-described polymer as a measuring reagent. As a result, they have completed the present invention.
In one aspect of the present invention, there is thus provided a method of, upon radical copolymerization of a vinyl-containing porphyrin compound and a radically polymerizable monomer, selecting the radically polymerizable monomer, thereby controlling the reactivity of the porphyrin-nucleus introduced polymer with a metal.
In another aspect of the present invention, there is also provided a trace metal measuring method by measuring a change in absorbance caused by a complex forming reaction between a porphyrin-nucleus introduced polymer, which has been obtained by radical copolymerization of a vinyl-containing porphyrin compound and a radically polymerizable monomer, and a heavy metal ion, said radically polymerizable monomer being selected so as to heighten reaction selectivity of the porphyrin-nucleus introduced polymer to the target metal.
In the trace metal measuring method according to the present invention, appropriate selection of the radically polymerizable monomer, which is a raw material for the porphyrin-nucleus introduced polymer serving as a measuring reagent, makes it possible to change water solubility of the reagent or reactivity with the metal upon complex formation, thereby facilitating measurement of the target metal with high sensitivity.